1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a facsimile machine, a printer, etc., and in particular to an image forming apparatus capable of determining an amount of transfer current supplied to a transfer nip in accordance with an image area rate of a latent image bearer in the vicinity of the transfer nip.
2. Description of the Background Art
An image forming apparatus capable of determining an amount of transfer current supplied to a transfer nip is described in Japanese Patent Application Laid Open No 8-83006. Such an image forming apparatus includes only one combination of a photoconductor and a transfer roller that contacts the photoconductor and creates a transfer nip therebetween, thereby forming a monochrome image on a recording sheet. A transfer bias having a polarity opposite to that of a normal charge of toner is applied to the transfer roller. A toner image on the photoconductor is transferred receiving the transfer bias onto a recording sheet launched into the transfer nip. A background not subjected to optical writing and a latent image section subjected thereto are each charged on the surface of the photoconductor in the same polarity as the normal charge polarity of the toner. A potential of the background is higher than that of the latent image section. As a result, a prescribed amount of a current is supplied between the transfer roller and the photoconductor due to separation electric discharge caused at an exit of the transfer nip therebetween. Specifically, a greater amount of current flows into the background due to having a higher potential than that of the latent image section on the photoconductor.
In such a situation, when an image area rate of a photoconductor is relatively low in the vicinity of an exit of the transfer nip, a greater amount of a current needs to be supplied from a power source than when it is relatively high. Because, if not, a prescribed amount of a current cannot be supplied to the latent image section, and accordingly a transfer problem of uneven image density occurs in accordance with the image area rate. Then, the image forming apparatus changes an output target amount of a current as time elapses to be supplied from the power source in accordance with the image area rate in the vicinity of the exit of the transfer nip to obtain stable image density regardless of the image area rate.
An image forming apparatus capable of forming a color image by superimposing toner images during transfer steps is known and described in Japanese Patent Application Laid Open No 2003-186284 (JP-2003-186284-A). Specifically, multiple toner images borne on latent image bearers, such as photoconductors, etc., are superimposed on a transfer member, such as an intermediate transfer member, etc., using various known techniques.
For example, the image forming apparatus of JP-2003-186284-A employs a tandem system to execute the superimposition of toner images. For this purpose, the image forming apparatus includes four photoconductors separately forming toner images of Y to Bk (magenta to black) monochrome colors. An intermediate transfer belt is employed contacting the photoconductors, to form primary transfer nips therebetween for the Y to Bk colors, respectively. Specifically, the Y toner image on the Y color photoconductor is transferred in the M color primary transfer nip onto the intermediate transfer belt in the Y color primary transfer nip. The M toner image on the M color photoconductor is transferred onto the Y color toner image borne on the intermediate transfer belt. Subsequently, C and Bk color toner images are similarly transferred and superimposed on the Y and M color toner images on the intermediate transfer belt in their primary transfer nips, respectively. A full-color image is finally formed on the intermediate transfer belt when such superimposition is completed in the transfer steps.
Another known type of an image forming apparatus includes a photoconductor, four developing devices for separately developing latent images formed on the photoconductors with toner of Y to Bk colors and superimposing the images on an intermediate transfer belt. Specifically, such an image forming apparatus forms toner images of different colors from each other on the photoconductor and superimposes those on an intermediate transfer belt with each rotation thereof to generate a color image on the intermediate transfer while circulating the belt about four times.
In the above-described systems, image density possibly fluctuates in accordance with an image area rate as in a monochrome type image forming apparatus as described in Japanese Patent Application Laid Open No 8-83006 (JP-8-83006-A). An experiment is performed using a tandem type color printer test machine on condition that a target amount of an output current supplied from each of primary transfer power sources of respective colors of Y to Bk is changed in accordance with an image area rate of a photoconductor of each of the respective colors. It is then confirmed that toner images of respective colors can be efficiently transferred onto an intermediate transfer belt from the photoconductors during their primary transfer processes. However, a significant amount of a toner image borne on the intermediate transfer belt is transferred reversely to a background section of the photoconductor in their downstream side primary transfer nip. For example, a large amount of a Y toner image is preferably transferred onto the intermediate transfer belt in the Y color primary transfer nip, but is transferred reversely on to backgrounds of the photoconductors in the other colors in their downstream side primary transfer nips. Similarly, a significant amount of an M color toner image on the intermediate transfer belt is transferred reversely onto backgrounds of the photoconductors of C and Bk colors in their downstream side primary transfer nips, respectively. Yet further, a C color toner image on the intermediate transfer belt is transferred reversely onto a background of the photoconductor of a Bk color in the downstream side primary transfer nip.
It is revealed that such reverse transfer is prominent when a region only including a background or a remarkably small image area rate (collectively referred to as a large image background region) among the entire region of the photoconductor in its rotational direction is positioned at the downstream side of the exit of the transfer nip.
However, such reverse transfer has not been induced in a conventional tandem type image forming apparatus even though the large area background region is positioned at the exit of the transfer nip. That is, the conventional tandem type image forming apparatus provides a prescribed constant transfer current regardless of an image area rate of the photoconductors. Specifically, when a large area background region of the photoconductor enters a transfer nip, a prescribed amount of a current flows in the transfer nip between the photoconductor and a belt, and accordingly a potential of the belt decreases. As a result, a potential difference between the background of the large area background region of the photoconductor and the intermediate transfer belt becomes smaller at the exit of the transfer nip, and electric discharge is hardly induced therebetween.
By contrast, a target amount of transfer current is changed in the above-described color printer testing machine to minimize fluctuation of image density caused in accordance with a change in an image area rate on a photoconductor in the vicinity of an exit of a transfer nip as in the past. Accordingly, when a large area background region of a photoconductor is positioned at the exit, the output target amount necessarily needs to be increased extraordinarily. As a result, electric discharge is actively induced between the photoconductor and the belt at the transfer nip exit thereby causing serious reverse transfer of the toner.
The similar problem possibly occurs in a circulation type image forming apparatus as in the above-described tandem type. Because, when second and subsequently color toner images are transferred onto the intermediate transfer belt at transfer nips during second and subsequently circulations thereof after transferring of a first color toner image thereonto, the toner on the belt is possibly transferred back onto a background of the photoconductor at a transfer nip exit.